1. Field of the Invention
Aspects of the present invention relate to a gap pull-in method and an optical disc apparatus therefor, and more particularly, to a method of performing gap pull-in more stably by preventing corruption with a disc in a near-field and an optical disc apparatus therefor.
2. Related Art
A near-field optical disc apparatus achieving large capacity and high data transfer rate (DTR) has recently been proposed. The near-field optical disc apparatus is also called a near-field recording system. The near-field optical disc apparatus records data on a disc or reproduces data from the disc using light in a near-field in which light diffraction does not occur. Therefore, the near-field optical disc apparatus needs to control such that a gap distance between an end surface of a solid immersion lens (SIL) mounted on a light focusing element, such as an object lens or the like, and an optical disc is very small, for example, several tens of nm. The gap distance is half of a wavelength of input laser light. In the case of a blue violet laser having a wavelength of 400 nm, the gap distance is approximately 200 nm.
In the near-field optical disc apparatus, when an actuator ascends and a near-field is formed, since an optical disc absorbs some incident light, an amount of reflected light decreases, thereby decreasing a gap error signal level. When the gap error signal level decreases to the target point, the near-field optical disc apparatus performs gap pull-in.
However, because the gap distance is very small, for example, several tens of nm as described above, when overshoot occurs during gap pull-in, the SIL may collide with a disc. The overshoot may occur, for example, due to decrease in the dynamic characteristics of the actuator or contamination of the SIL. In addition, as each disc has different reflectivity, the gap error signal changes, and stability is reduced when controlling gap servo depending on the gap distance.